Binding of misonidazole to EMt6 and V79 spheroids.

Metabolism-induced binding of misonidazole was shown previously to be enhanced in hypoxic regions of V79 spheroids. Data are presented to show that the binding of misonidazole to EMT6 spheroids is similar to V79 in some respects and different in others. The binding rate in nitrogen was constant in all cells in EMT6 spheroids, whereas a 5-fold rise was found in binding rate across the outer 100 micrometers of V79 spheroids. A small enhancement of binding to the cells near the necrotic center in EMT6 spheroids in air was found, which was similar to the binding to chronically hypoxic cells in aerobic V79 spheroids. The binding rate in 1--5% oxygen was similar in EMT6 and V79 spheroids. Both showed a large rise in binding rate across the outer 120 micrometers and a constant binding rate to healthy cells interior to this. Since 1.5 oxygen should supply only the outer 40 micrometers of a typical spheroid, something beyond a simple oxygen diffusion limitation is required to explain the data. One possible explanation is the diffusion of some of the reactive product away from the hypoxic cells which produce it. This was tested by incubating a single cell suspension in misonidazole at 0.5% oxygen in the presence of absence of V79 spheroids. No evidence was found for an appreciable loss of reactive product from the spheroids.